Copper is a good electrical conductor and comparatively inexpensive. Copper interconnects are commonly used in semiconductor printed circuit boards and package mounts for integrated circuits. Recently, copper has also been used as both gate and interconnect layers to increase the speed of silicon integrated circuits.
In the fabrication of integrated circuits, complex interconnects are required to couple together different devices. However, a series of undesirable non-planar steps may be formed on the chip by the interconnects. To solve this problem, interconnect metallization layers are typically coated with a deposited dielectric layer and then the surface is planarized by using a chemical mechanical polishing process.
Chemical mechanical polishing (CMP) is becoming a more common process step for planarizing semiconductor wafers, although it can also be used to polish and planarize other surfaces as well. A CMP planarization process typically uses a polishing slurry comprised of a chemical etchant and an abrasive component, such as alumina or silica particles. In a CMP planarization process, the polishing pad type, force between pad and wafer, and slurry composition are adjusted so that raised surface features on the wafer that are in physical contact with the polishing pad experience greater mechanical etching from the abrasive components of the slurry.
The abrasive silica or alumina particles used in common CMP polishing slurries typically have a particle size in the range of a few tenths of a micron to several microns in diameter. After a chemical mechanical polishing process, the wafer is readily cleaning of the bulk film of polishing slurry coating the wafer during the CMP process. However, a microscopic surface layer of residual slurry particles may still remain on portions of the wafer surface even after the bulk film of slurry is rinsed away. These residual slurry particles often interfere with subsequent device processing and may reduce device reliability. Consequently, residual slurry particles that remain on the wafer surface after CMP are highly undesirable.
Subsequent to a CMP planarization process, a wafer needs to be cleaned of deleterious residues and contaminants from the CMP process. Post-CMP cleaning procedures used for silicon wafers coated with dielectric materials or common metallization layers include de-ionized water with citric acid or potassium hydroxide, with the option of adding surfactants. However, post-CMP cleaning processes for copper interconnects need to be developed that address the special problems of copper surfaces that are polished with a CMP process.
Experiments by the inventors of the herein described cleaning method using scanning electron microscopy, Auger, and energy dispersive x-ray analysis indicate that residual alumina particles remain adhered on a copper surface after a CMP process using an alumina slurry. The inventors have also determined that these alumina residues are a source of interconnect failure. However, the inventors have experimentally determined that many common cleaning procedures are ineffective at removing alumina particles from a copper surface subsequent to a CMP process utilizing an alumina slurry. Brushing while rinsing with water does not remove residual alumina particles from a copper surface. Brushing the wafer with a 6% citric acid solution was found to be ineffective for removing alumina particles from a copper surface. Similarly, brushing a copper surface with a KOH solution was also found to be ineffective for removing alumina particles.
The inventors believe that alumina particle residues which remain on copper surfaces subsequent to a CMP process may cause both reliability and yield problems. The inventors have determined that alumina particle residues that are left on copper interconnects tend to cause breakage of copper interconnects. Alumina particle residues are thus a potential reliability problem for copper interconnects. Additionally, embedded slurry particles tend to reduce the yield of subsequent semiconductor fabrication processes. The inventors' experiments indicate that alumina particle residues may induce cracks in subsequently deposited thin (e.g., 200 Angstroms) seed layers which are commonly used for electrodeposition processes, thereby lowering the yield of such processes.
A post-CMP cleaning process for copper interconnects should effectively remove residual slurry particles from all copper surfaces, but without stripping the interconnect metalization. Moreover, such a cleaning process should be consistent with a high-yield interconnect fabrication process. However, experiments by the inventors indicate that previously known surfactants and rinses which are commonly used to clean wafers after a CMP process are ineffective at removing residual alumina particles from a copper surface.
What is desired is a new method to effectively remove residual slurry particles from copper surfaces after a CMP planarization process performed on a wafer with copper interconnects.